Conventional practices for preventing air from leaking from a pneumatic radial tire including at least one carcass layer include to increase the thickness of an inner liner layer which is disposed in the innermost surface of the tire, or to use a material with a smaller air permeability coefficient for the inner liner layer. However, in a case where the inner liner layer is made thicker, the tire weighs heavier, and the rolling resistance of the tire worsens. Meanwhile, in a case where a material with a smaller air permeability coefficient is used for the inner liner layer (see Patent Document 1, for example), it is possible to prevent air from leaking from the tire without increasing the weight of the tire. However, once air having permeated the inner liner layer reaches the carcass layer, the air quickly propagates to the wound-up portions of the carcass layer, and leaks out from the wound-up portions to the outer surface of the tire. Generally speaking, that is because the air permeability coefficient of the carcass cords which are twisted cords is extremely larger than that of any rubber composition.
With this taken into consideration, for the purpose of effectively improving the internal pressure holding performance, it is effective to dispose a material having a smaller air permeability coefficient in the sidewall parts largely contributory to air leakage. As a concrete scheme, there has been proposed that an air permeation prevention layer made of a rubber composition mainly containing polyisobutylene isoprene rubber should be provided in the sidewall parts (see Patent Document 2, for example). However, the adhesiveness between polyisobutylene isoprene rubber and a diene rubber generally used for the sidewall parts is poor, and therefore, the disposition of the air permeation prevention layer made of the rubber composition mainly containing the polyisobutylene isoprene rubber in the sidewall parts, where large deformation of bending occurs, causes a serious problem from a viewpoint of the durability of the tire.
In addition, there have been proposed a structure in which a material having a smaller air permeability coefficient is disposed between the carcass layer and a reinforcement member such as a chafer in each bead part (see Cited Document 3, for example), and a structure in which the inner liner layer is disposed in a way that a part of the inner liner layer is folded back to reach a position below each bead core (see Patent Document 4, for example). However, even when these structures are used, it is still difficult to prevent air from leaking out from each wound-up portion of the carcass layer to the outer surface of the tire. In addition, the problem with the adhesiveness between the air permeation prevention layer made of the rubber composition mainly containing the polyisobutylene isoprene rubber and its surrounding rubber layer remains yet to be solved.    Patent Document 1: Japanese patent application Kokai publication No. Hei. 11-123907    Patent Document 2: Japanese patent application Kokai publication No. 2000-190713    Patent Document 3: Japanese patent application Kokai publication No. Sho. 62-139705    Patent Document 4: Japanese patent application Kokai publication No. Hei. 11-320705